I would like to give you every ounce of my gratitude
And send it to you through this song
This is a feeling I will never, ever forget

Lyrics from Animelyrics.com

omoi yo todoke[1]

Lyrics from Animelyrics.com

Oh feelings, please get through

With corrections by Serei.There are many ways to interpret the song title from the surface, but obviously as we saw from events in Episode 17, it is an allusion to the name of Yui's little sister Ui.
Other interpretations include:
You and I
Yui herself (English "You" + Japanese "I")
'yuuai', "partnerly love" (Japanese phonetic interpretation of the English letters)
Even 'utoi', "estranged" (Replacing & with 'to', Japanese particle for "and").
My own overly-fanciful suggestion is to take the on-reading of Ui's name kanji (憂, 'yuu', "grief"), add it with the radical for 'person' (as the "I") which will become 優, the kanji for "gentleness", Ui's character in whole. (This is ultimately why Ui was given that "depressing" kanji as her name in the first place, she "warms up to people around her").